<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
      .parent{
        width: 200px;
        height: 200px;
        background-color: skyblue;
        display:flex;
        justify-content: center;
        align-items:center;
      }
      .parent .first{
        width: 50px;
        height: 50px;
        background-color: red;
        text-align:center;
        line-height:50px;
      }
  </style>
</head>
<body>
  <div class="parent">
    <div class="first">1</div>
  </div>
</body>
</html>